1. Field of the Invention
The present invention relates to an ink-jet printhead for use in an ink-jet printer or a facsimile, and more particularly, to a thermal ink-jet printhead.
2. Description of the Related Art
In a general thermal ink-jet printhead, ink filled in an ink chamber is rapidly heated by using a heater to generate a bubble, and a droplet of the ink is ejected onto a print medium by the expansive force of the bubble to form an image on the print medium.
Meanwhile, the thermal ink-jet printhead may be classified into an edge-shooter type ink-jet printhead, a roof-shooter type ink-jet printhead, and a back-shooter type ink-jet printhead according to an ink ejecting method.
In the edge-shooter type ink-jet printhead, as described in U.S. Pat. No. 4,490,728, to Vaught et al., issued Dec. 25, 1984, the ink is introduced into the ink chamber in an ink introducing direction parallel to a surface of the heater (i.e., the ink introducing direction for passing through a side of the ink chamber), and then ejected through a nozzle in an ink ejecting direction parallel to the surface of the heater. In the edge-shooter type ink-jet printhead, since the ink introducing direction for introducing the ink into the ink chamber coincides with the ink ejecting direction for ejecting the ink through the nozzle, there is an advantage in that the ink is introduced into the ink chamber and stably ejected through the nozzle. However, there is also a disadvantage in that productivity of the ink-jet printhead is reduced. That is, in order to fabricate the edge-shooter type ink-jet printhead, the heater is formed on a substrate, and then an attachment process is performed twice to attach an ink chamber barrier layer for forming the ink chamber on the substrate and a nozzle plate, in which the nozzle is formed, in turn.
In the roof-shooter type ink-jet printhead, as described in U.S. Pat. No. 6,060,208, to Wang, issued May 9, 2000, the ink is introduced into the ink chamber in the ink introducing direction parallel to a surface of the heater, and then, ejected through the nozzle in the ink ejecting direction vertical to the surface of the heater. In the roof-shooter type ink-jet printhead, the ink chamber is formed on the nozzle plate. Then, the substrate on which the nozzle plate and the heater are formed is attached. Therefore, since the attaching process is performed only once to fabricate the ink-jet printhead, there is an advantage that the productivity is higher than that of the edge-shooter type ink-jet printhead. However, since the ink introducing direction into the ink chamber is vertical to the ink ejecting direction through the nozzle, there is a disadvantage that the ink is unstably ejected.
Further, in the back-shooter type ink-jet printhead, as described in U.S. Pat. No. 5,760,804, to Heinzi et al., issued Jun. 2, 1998, the ink is passed through the heater in the direction vertical to the surface of the heater and then ejected. As shown in FIG. 1, the back-shooter type ink-jet printhead includes a substrate 1, a doughnut-shaped heater 2 formed on an upper surface of the substrate 1 and having an opening at a center portion thereof, and a nozzle plate 3 stacked on an upper surface of the heater 2.
The substrate 1 is provided with an ink chamber 4 formed below the heater 2 and an ink passage 5 communicating with the ink chamber 4. The nozzle plate 3 has a nozzle 3a communicating with the opening of the heater 2. The nozzle 3a, the opening of the heater 2, the ink chamber 4 and the ink passage 5 are concentrically communicating with each other. The ink is introduced through the ink passage 5 into the ink chamber 4, and then ejected through the nozzle 3a in the ink ejecting direction vertical to the surface of the heater 2. As described above, in the back-shooter type ink-jet printhead, the ink introducing direction coincides with the ink ejecting direction. A reference numeral 6 is a bubble generated by heating the heater 2.
Generally, the back-shooter type ink-jet printhead is fabricated without using the attaching process by a monolithic method, which is different from the edge-shooter type or roof-shooter type ink-jet printhead. First of all, the heater 2 is formed on the substrate 1. The nozzle plate 3 is stacked thereon by a chemical vapor deposition (CVD) method. Then, the nozzle 3a is formed in the nozzle plate 3. The heater 2 is etched through the nozzle 3a to form the opening at the center portion of the heater 2. The substrate 1 is etched to form the ink chamber 4 and the ink passage 5 in turn. The back-shooter type ink-jet printhead has a high productivity compared with the edge-shooter type or roof-shooter type ink-jet print head since the attaching process is not required to form the nozzle plate 3 or the ink chamber 4 in the monolithic process.
However, in the back-shooter type ink-jet printhead shown in FIG. 1, since a path along which the heat generated from the heater 2 is conducted is remarkably shorter than that in the edge-shooter type or roof-shooter type ink-jet printhead, the cooling rate of the heater 2 is low. In the ink-jet printhead, the number of ink droplets that can be ejected per hour, i.e., the ejection frequency, depends on the cooling rate of the heater 2. The low cooling rate of the heater 2 reduces the ejection frequency of the ink and results in the low print speed of the printer.
Further, in the back-shooter type ink-jet printhead, since the nozzle plate 3 is formed on the substrate 1 by using the CVD method, the thickness of the nozzle plate 3 is less than that (above about 10 μm) in the edge-shooter type or roof-shooter type ink-jet printhead. The strength of the nozzle plate 3 decreases. Further, in the back-shooter type ink-jet printhead, since the heater 2 is formed in the lower portion of the nozzle plate 3, the nozzle plate 3 is prone to be contaminated by the ink sludge.
Accordingly, the nozzle is required to be wiped and cleaned more frequently than that of the edge-shooter type or roof-shooter type ink-jet printhead. In addition, in the back-shooter type ink-jet printhead, nevertheless the ink introducing and ejecting directions coincide with each other, it is observed that the ejection of the ink is less stable than in the edge-shooter type or the roof-shooter type ink-jet printhead. This is because the thickness of the nozzle plate 3 is too small to become uniform or the shape of the nozzle 3a cannot be ideally formed.